(1) Field of the Invention
The present invention relates to suspension type lightning-conducting insulators which can speedily discharge, to the earth, a surge current applied to a power transmission line due to lightning, interrupt a follow current, prevent a problem that the power transmission line contacts the ground, and provide power to be transmitted again.
(2) Related Art Statement
Heretofore, it has been investigated to introduce lightning-conductors into power transmission lines so as to absorb the lightning and ON-OFF surges and prevent the ground-contacting problems of the power transmission lines. However, it is undesirable to attach lightning conductors having only a lightning-conducting function to the power transmission lines because the structure of iron towers for the power transmission lines and the insulating units becomes complicated. For this reason, suspension type lightning-conducting insulators have been proposed which have insulating and power transmission line-supporting functions as possessed by the conventional insulators and a lightning-conducting function of the lightning-conductors in combination. As such lightning-conducting insulators, NGK Insulators, Ltd. and The Tokyo Electric Co., Ltd. proposed lightning-conducting insulators in which a current-limiting element having a non-linear voltage-current characteristic is attached to a shed portion of an insulator body. In these lightning-conducting insulators, the current-limiting element is housed in a cylindrical fitting section, and an upper electrode and a lower electrode, each made of a cap-like metal member, are fitted and fixed to upper and lower ends of the cylindrical fitting section through packings. Further, the upper electrode is connected to a metal cap by a lead line, while the lower electrode is connected to a metal pin by a lead wire.
However, since the conventional lightning-conducting insulators fixed the upper and lower metallic electrodes to the cylindrical fitting section through the packings, the following problems were encountered.
It was difficult to maintain a gas-tightness of the inside of the cylindrical fitting section due to deterioration of the packing during use for an extended time period. When the gas-tightness is lowered, the current-limiting element is damaged so that it may become conducting when struck by lightning. Consequently, it becomes impossible to interrupt follow current subsequent to the lightning surge, resulting in the ground-contacting problems.
Furthermore, there have been proposed lightning-conducting insulators in which an insulating ceramic member is fitted to the cylindrical fitting section and lead wires are taken out from the current-limiting element therethrough. According to such lightning-conducting insulators, while adhesion between the cylindrical fitting section and the insulating ceramic member is good, it is necessary to make a cut portion through which the lead wires pass in the above insulating ceramic member so that a working operation is not only troublesome, but also a special technique is necessary for sealing the cut portion. Thus, a special contrivance was necessary for maintaining the reliability.